Olaf Sporns, one of two co-coiners of the term “connectome” and an investigator in the Human Connectome Project, is well-positioned to speak on the advances in the field of neuroanatomy over the last twenty years. In Networks of the Brain, published last fall from MIT press, he does exactly that.
Terrence Sejnowski recently reviewed Networks of the Brain for American Scientist Online. In so doing, Sejnowski offers a succinct review of the advances in this field, such as diffusion tensor imaging modeling of white matter fibers, producing images that have become nearly synonymous with the Human Connectome Project. However, the focus of this book, and Sejnowski’s review of it, is not directly related to any particular imaging modality. Rather, the more abstract field of network science receives the most attention.
We have known for some time that there are relatively few long-range connections in the brain, and for good reason: If all 10 billion cortical neurons in the human cerebral cortex were connected with each other, the brain would be the size of a football stadium. As a consequence, the brain’s connectivity pattern is locally dense and globally sparse, following a pattern called a small-world network. The small-world effect was first documented empirically in social-network experiments by Stanley Milgram in the 1960s, and it gave rise to the idea that you are connected to anyone else in the world by no more than “six degrees of separation” through a chain of mutual acquaintances. More detailed analysis of brain structures has revealed interlocking clusters of areas that are responsible for the major brain functions, such as our sensory, motor and planning systems.
In addition to Sporns’ book, several other HCP publications tackle this very subject. These include Tim Behren’s and Olaf Sporns’ “Human Connectomics,” published in Current Opinions in Neurobiology, and “Functional Network Organization of the Human Brain” by Jonathan Power et al, published in Neuron.
Of course, even if we had a complete “wiring diagram” of the brain, the work of connectomics would be in no way complete. As Sejnowski writes: “Creating a complete map of the human connectome would be a monumental milestone but not the end of the journey to understanding how our brains work. The achievement will transform neuroscience and serve as the starting point for asking questions we could not otherwise have answered.”